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@nativescript-community/ui-htmlcanvasapi

An HTML Canvas implementation on top of @nativescript-community/ui-canvas.

Supported classes:

  • OffscreenCanvas
  • CanvasRenderingContext2D
  • ImageBitmapRenderingContext
  • OffscreenCanvasRenderingContext2D
  • CanvasGradient
  • CanvasPattern
  • DOMMatrix
  • DOMPoint (not used yet)
  • Path2D
  • TextMetrics

Installation

npm install @nativescript-community/ui-htmlcanvasapi

Usage

Install polyfills (optional)

import { Application } from '@nativescript/core';
import { installPolyfills } from '@nativescript-community/ui-htmlcanvasapi';

installPolyfills();

Application.run({ moduleName: 'app-root' });

Then, set up your view

<Page xmlns="http://schemas.nativescript.org/tns.xsd" xmlns:canvas="@nativescript-community/ui-htmlcanvasapi">
  <Page.actionBar>
    <ActionBar title="ui-htmlcanvasapi" icon="" class="action-bar">
    </ActionBar>
  </Page.actionBar>
  <StackLayout class="p-20">
    <canvas:HTMLCanvasView width="400" height="400" hardwareAccelerated="true" draw="onDraw"/>
  </StackLayout>
</Page>
import { Canvas } from '@nativescript-community/ui-canvas';
import { HTMLCanvasView } from '@nativescript-community/ui-htmlcanvasapi';

export function onDraw(args: { object: HTMLCanvasView; canvas: Canvas }) {
	const ctx = args.object.getContext('2d');

	ctx.save();

	ctx.fillStyle = 'yellow';
	ctx.fillRect(10, 10, 200, 100);

	ctx.transform(1, 0.5, -0.5, 1, 30, 10);

	ctx.fillStyle = 'red';
	ctx.fillRect(10, 10, 200, 100);

	ctx.transform(1, 0.5, -0.5, 1, 30, 10);

	ctx.fillStyle = 'blue';
	ctx.fillRect(10, 10, 200, 100);

	ctx.restore();
}

Note: If you wish to draw outside of draw listener context, you should call HTMLCanvasView invalidate() method in order to request view to redraw itself.

import { HTMLCanvasView } from '@nativescript-community/ui-htmlcanvasapi';

function updateGraph(canvasView: HTMLCanvasView) {
	const ctx = canvasView.getContext('2d');

	ctx.save();
	ctx.fillStyle = 'yellow';
	ctx.fillRect(10, 10, 200, 100);
	ctx.restore();

	canvasView.invalidate();
}

Sometimes, there might be a need to draw things offscreen but keep reference to the view and eventually want to draw everything there.
HTMLCanvasView includes these custom methods:

  • enableOffscreenBuffer
  • disableOffscreenBuffer
  • isOffscreenBufferEnabled
  • drawOffscreenBuffer
const view = args.object;
const ctx = view.getContext('2d');

view.enableOffscreenBuffer();

ctx.save();
ctx.fillStyle = 'yellow';
ctx.fillRect(10, 10, 200, 100);
ctx.transform(1, 0.5, -0.5, 1, 30, 10);
ctx.fillStyle = 'red';
ctx.fillRect(10, 10, 200, 100);
ctx.transform(1, 0.5, -0.5, 1, 30, 10);
ctx.fillStyle = 'blue';
ctx.fillRect(10, 10, 200, 100);
ctx.restore();

view.drawOffscreenBuffer();
// If buffer is no longer needed, just discard it
view.disableOffscreenBuffer();

Note: This state is required to perform actions like toDataURL.

License

Apache License Version 2.0